1. Field of the Invention
The present invention relates to a solar cell lead wire, and particularly to a solar cell lead wire with a high cell cracking inhibiting effect and a production method therefor and a solar cell using the lead wire.
2. Description of the Related Art
In solar cells, as semiconductor substrates, there are used polycrystalline and monocrystalline Si cells.
As shown in FIGS. 6A and 6B, a solar cell 100 is fabricated by solder-bonding solar cell lead wires 103a and 103b to specified regions of semiconductor substrate 102, i.e., to frontside and backside electrodes 104 and 105 provided on the frontside and backside, respectively, of the semiconductor substrate 102. Power generated in the semiconductor substrate 102 is transmitted outside through the solar cell lead wire 103.
As shown in FIG. 7, the conventional solar cell lead wire 103 comprises a strip-shaped conductive material 112 and melt solder-plated layers 113 formed over the upper and lower surfaces, respectively, of the strip-shaped conductive material 112. The strip-shaped conductive material 112 is formed by rolling a conductor with a circular cross section into a strip shape, and is also referred to as a rectangular conductor or a rectangular wire.
The melt solder-plated layers 113 are formed by supplying melt solder (melt plating) to the upper and lower surfaces of the strip-shaped conductive material 112.
The melt plating method cleans the upper and lower surfaces 112a and 112b of strip-shaped conductive material 112 with acid or the like, passes the strip-shaped conductive material 112 through a melt solder bath, and thereby stacks solder on the upper and lower surfaces 112a and 112b of the strip-shaped conductive material 112. As shown in FIG. 7, the melt solder-plated layers 113 are formed to be rounded from sides in width direction to the middle, by surface tension when melt solder adhering to the upper and lower surfaces 112a and 112b of the strip-shaped conductive material 112 is solidified.
The conventional solar cell lead wire 103 shown in FIG. 7 has the rounded melt solder-plated layers 113 formed on the upper and lower surfaces 112a and 112b of the strip-shaped conductive material 112. Since this solar cell lead wire 103 has the rounded melt solder-plated layers 113, when wound around a bobbin, the stacked layer is unlikely to be stable, and tends to unwind. The lead wire may be tangled and not drawn due to unwinding.
This solar cell lead wire 103 is cut at specified length, air-sucked, transferred and soldered to the frontside electrode 104 of the semiconductor substrate 102. The frontside electrode 104 is formed beforehand with an electrode strip (not shown) conducting with the frontside electrode 104. This frontside electrode 104 is contacted with melt solder-plated layers 113 of solar cell lead wire 103a, followed by soldering in that state. The same applies to soldering of solar cell lead wire 103b to backside electrode 105 of the semiconductor substrate 102.
In this case, since melt solder-plated layers 113 in solar cell lead wire 103 of FIG. 7 is rounded and uneven in thickness, there is the problem of insufficient suction due to small contact area with an air suction jig, and therefore falling during transfer. Also, the contact area between frontside electrode 104 and melt solder-plated layers 113 is small. The small contact area between the frontside electrode 104 and melt solder-plated layer 113 causes insufficient thermal conduction from semiconductor substrate 102 to melt solder-plated layer 113, and therefore poor soldering.
Also, when bonding the solar cell lead wires 103a and 103b to both the frontside and backside of the semiconductor substrate 102, the small contact area between the frontside electrode 104 and melt solder-plated layer 113 causes a position shift between the solar cell lead wire 103a soldered to the frontside electrode 104 and the solar cell lead wire 103b soldered to the backside electrode 105. That position shift causes cell cracking (semiconductor substrate 102 cracking). Because the semiconductor substrate 102 is expensive, cell cracking is undesirable.
To solve the problem of the small contact area between the frontside electrode 104 and the melt solder-plated layer 113, there has been suggested a method of forming a depressed lower surface in strip-shaped conductive material, supplying melt solder to the depressed lower surface and thereby forming a flat melt solder-plated layer (See International Publication WO2004/105141).
As shown in FIG. 8, a solar cell lead wire 203 disclosed by International Publication WO2004/105141 uses a strip-shaped conductive material 212 formed with depressed lower surface 212b. Upper surface 212a of the strip-shaped conductive material 212 is rounded or flat. The strip-shaped conductive material 212 which is depressed at only its lower surface 212b in this manner is passed through a melt solder bath, to thereby form melt solder-plated layers 213 and 214 over the upper and lower surfaces 212a and 212b, respectively, of the strip-shaped conductive material 212. The melt solder-plated layer 214 at the depressed lower surface 212b of the strip-shaped conductive material 212 is flat.
Soldering a flat lower surface 214b of the melt solder-plated layer 214 of such solar cell lead wire 203 to a frontside or backside electrode of a semiconductor substrate allows the solar cell lead wire 203 to be firmly bonded to the semiconductor substrate, and be unlikely to be separate therefrom, resulting in excellent durability.
As described above, to firmly bond the solar cell lead wire to the semiconductor substrate, it is preferable to form flat the melt solder-plated layers 113 and 214. However, according to International Publication WO2004/105141, to form the depressed lower surface 212b in the strip-shaped conductive material 212, the strip-shaped conductive material 212 is deformed or bent appropriately. For example, the strip-shaped conductive material 212 is passed around a mold roll to thereby form a depressed surface. Also, when a flat sheet cladding material is slit to provide a strip-shaped conductive material, a rotary blade pitch or rotary speed is adjusted for bending. In this manner, the strip-shaped conductive material 212 with the depressed lower surface is provided.
However, the deforming or bending is intermittent, and therefore poor in mass productivity. Also, passing the strip-shaped conductive material 212 around a mold roll leads to poor accuracy in cross section dimensions of the strip-shaped conductive material 212 with the depressed lower surface because of difficult pressure adjustment to the strip-shaped conductive material 212.
When the strip-shaped conductive material 212 is slit to form a depressed surface therein, burr occurs at both sides of the lower surface 212b of the strip-shaped conductive material 212. The burr present on the strip-shaped conductive material 212 and the thin melt solder-plated layer 214 thereon, when the solar cell lead wire 203 is bonded to the semiconductor substrate 102, causes the burr and cell to come into contact due to melting of the solder-plated layer 214, leading to stress concentration in the contact portion of the burr and cell, and therefore cell cracking in the semiconductor substrate 102.
Also, in the solar cell lead wire 203 of International Publication WO2004/105141, connection is made from the backside electrode of the first semiconductor substrate to the frontside electrode of the second semiconductor substrate, and from the backside electrode of the second semiconductor substrate to the frontside electrode of the third semiconductor substrate. When in this manner the solar cell lead wire 203 is bonded to both the frontside and backside of the semiconductor substrate, the problem is unsolved of causing a position shift between the solar cell lead wire 203 soldered to the frontside electrode and the solar cell lead wire 203 soldered to the backside electrode. There remains the problem of this position shift causing cell cracking in the semiconductor substrate.
Further, even if the plated layer is flat and no burr is present, if the conductor has a corner as in a rectangular parallelepiped shape, the conductor leans during solder melting, to cause a contact between the corner of the conductor and the cell to cause stress concentration at the contact portion therebetween to cause cell cracking in the semiconductor substrate.
The semiconductor substrate is made thin because of occupying most of the cost of the solar cell. However, the thin semiconductor substrate tends to crack. For example, for the semiconductor substrate being at thicknesses of not more than 200 μm, the cell cracking rate is large. The thin semiconductor substrate, in which cell cracking is caused by the solar cell lead wire, is not desired.